The present invention relates to high-strength, high-modulus, melt-processed microfibers, films having a microfibrillated surface, and methods of making the same. Microfibers of the invention can be prepared by imparting fluid energy, typically in the form of ultrasound or high-pressure water jets, to a highly oriented, highly crystalline, melt processed film to liberate microfibers therefrom. Microfibrillated films of the invention find use as tape backings, filters, fibrous mats and thermal and acoustical insulation. Microfibers of the invention, when removed from the film matrix, find use as reinforcement fibers for polymers or cast building materials such as concrete.
Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. In particular, the ease with which films produce fibers (i.e., fibrillate) can be correlated to the degree of molecular orientation of the polymer fibrils that make up the film.
Orientation of crystalline polymeric films and fibers has been accomplished in numerous ways, including melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.
Liberating fibers from oriented, high-modulus polymer films, particularly from high molecular weight crystalline films, has been accomplished in numerous ways, including abrasion, mechanical plucking by rapidly-rotating wire wheels, impinging water-jets to shred or slit the film, and application of ultrasonic energy. Water jets have been used extensively to cut films into flat, wide continuous longitudinal fibers for strapping or reinforcing uses. Ultrasonic treatment of oriented polyethylene film in bulk (that is, a roll of film immersed in a fluid, subjected to ultrasonic treatment for a period of hours) has been shown to produce small amounts of microfibrils.
The present invention is directed to novel highly oriented, melt processed polymeric microfibers having an effective average diameter less than 20 microns, generally from 0.01 microns to 10 microns, and substantially rectangular in cross section, having a transverse aspect ratio (width to thickness) of from 1.5:1 to 20:1, and generally about 3:1 to 9:1. Since the microfibers are substantially rectangular, the effective diameter is a measure of the average value of the width and thickness of the microfibers.
The rectangular cross-sectional shape advantageously provides a greater surface area (relative to fibers of the same diameter having round or square cross-section) making the microfibers (and microfibrillated films) especially useful in applications such as filtration and as reinforcing fibers in cast materials. The surface area is generally greater than about 0.25 m2/gram, typically about 0.5 to 30 m2/g. Further, due to their highly oriented morphology, the microfibers of the present invention have very high modulus, for example typically above 109 Pa for polypropylene fibers, making them especially useful as reinforcing fibers in thermoset resin and concrete.
The present invention is further directed toward the preparation of highly-oriented films having a microfibrillated surface by the steps of providing a highly oriented, semicrystalline polymer film, stretching the film to impart a microvoided surface thereto, and then microfibrillating the microvoided surface by imparting sufficient fluid energy thereto. Optionally the microfibers may be harvested from the microfibrillated surface of the film.
Advantageously the process of the invention is capable of high rates of production, is suitable as an industrial process and uses readily available polymers. The microfibers and microfibrillated articles of this invention, having extremely small fiber diameter and both high strength and modulus, are useful as tape backings, strapping materials, films with unique optical properties and high surface area, low density reinforcements for thermosets, impact modifiers or crack propagation prevention in matrices such as concrete, and as fibrillar forms (dental floss or nonwovens, for example).